Several techniques exist for interfacing integrated circuits and other digital electronic devices with one another in high-performance, transmission line environments. Some of these use load termination to eliminate unwanted signal reflections that occur because of impedance mismatches along the transmission line. Standard interfaces such as GTL (Gunning Transceiver Logic) and PECL commonly use this kind of transmission. Another technique that offers some advantages over this accepted method of termination is the source-terminated driver method. A source-terminated line does not use load termination. Instead, a source-terminated line relies upon reflections off an unterminated end-of-line to reach a valid voltage level and then terminates the reflected waves back at the source.
Source-terminated lines and busses may be difficult to probe because the electrical signals can appear distorted if viewed or probed at a location other than the receiving end of the bus. This distortion is caused by the separation between the incident and reflected waves of logic transition traveling on the line. This distortion can also cause glitches and timing uncertainty in the detected version of the probed signal's logic transitions.
Accordingly, there is a need in the art for a probe circuit that can reliably detect the incident wave of a logic transition on a source-terminated bus. By detecting the incident wave and ignoring any reflections, the timing of the logic transition can be measured with greater certainty than detecting both the incident wave and possibly some reflected waves. Furthermore, detecting just the incident wave prevents reflected waves from possibly being detected as glitches on the line.